Accurate measurement of the relative bond energies of CO and H2O ligands in Fe+mono- and bis-ligated complexes

The systems Fe(H2O)n+/COH2O and Fe(CO)n+/COH2O (n = 1 and 2) were investigated in a triple cell Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Using mixtures of CO with a very small amount of water, the ligand exchange equilibrium was reached, allowing experimental determination of the relevant equilibrium constants and free energies of reaction. Quantum chemical calculations at the B3LYP level of theory on the reactant and product species allowed us to determine the entropic terms and to derive the relative bond energies of CO and H2O in the mono- and bis-ligated complexes. For n = 1, H2O is more strongly bound to Fe+ than CO by 4.1 ± 1.6 kJ·mol−1 at 298 K. For n = 2, at the same temperature, H2O is more strongly bound than CO to (H2O)Fe+ by 7.6 ± 1.6 kJ·mol−1, and to (CO)Fe+ by more than 20.1 kJ·mol−1. Copyright © 2003 John Wiley & Sons, Ltd.